[analyzer] Small SMT API improvement

Summary: Removed const qualifier from reset method of SMTSolver and Z3Solver objects.

Reviewers: mikhail.ramalho, george.karpenkov, NoQ, ddcc

Reviewed By: NoQ

Subscribers: xazax.hun, szepet, a.sidorin, Szelethus

Differential Revision: https://reviews.llvm.org/D52031

llvm-svn: 344463